Important question are you licensed for Storage vMotion - that will make the whole process much easier - I do not think there is any benefit upgrading to vSPhere 5 first - Since you existing environment is working I would do the storage migration under vSphere 4 (ESX 4) once that is complete then do the upgrade vSphere 5(ESXi5)
And you do realize moving from ESX to ESXi its is a fresh install -